[llvm-dev] load instruction erroneously removed by GVN

[Nick Lewycky via llvm-dev]

On 7 August 2015 at 05:02, Mikael Holmén <llvm-dev at lists.llvm.org> wrote:

>
>
> On 08/07/2015 01:53 PM, Caldarale, Charles R wrote:
>
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org]
>>> On Behalf Of Mikael Holmén via llvm-dev
>>> Subject: [llvm-dev] load instruction erroneously removed by GVN
>>>
>>
>> But between the load and the alloca there is also
>>>     call fastcc void @format_long(i16* %_tmp30, i16 10, i32 10), !dbg !22
>>> which will use %_tmp30 to write in the alloca'd buffer.
>>>
>>
>> Shoulnd't MemoryDependenceAnalysis::getDependency rather return the call?
>>>
>>
>> Depends.  What is the exact declaration of format_long?
>>
>
> In the input .ll file it is:
>
> ; Function Attrs: minsize optsize
> define internal i16 @format_long(i16* %res.8.par, i16 %base.9.par, i32
> %x.10.par) #3 {
>
> which is later changed somewhere in opt to:
>
> ; Function Attrs: minsize nounwind optsize
> define internal fastcc i16 @format_long(i16* %res.8.par, i16 %base.9.par,
> i32 %x.10.par) #2 {
>

That's part of it, but we also need to see what #3 and #2 refer to in this
context. Those should be grouped at the end of your .ll, something like

attributes #3 = { nobuiltin nounwind "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-nans-fp-math"="false"
"stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
"use-soft-float"="false" }

or

attributes #2 = { nounwind readonly }

If @format_long is marked with the 'readonly' or 'readnone' attribute then
GVN would be justified in doing the transformation you've described.

Nick
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